What is the significance of the horn of an Ox that the Greeks made the Jews write (Midrash Rabbah 2:5 and other places) We have no Part in the God of Israel on it (even thought in his Epistle to Yemen the Rambam says they wrote on their Clothes)? What is the Significance of the ox in this context?

Kings David and Shlomo were annointed with horns, and we pray 3 times a day "Es Keren David Avdecha B'Meheira Tazmiach" "The offspring of Your servant David, (revelation of Mashiach) may You speedily cause to flourish."

The Greeks were trying to get the Jews to deny the coming of Moshiach. (B'nei Yissachar)

With regards to an animal goring, Torah law has differences between whether the animal goring belongs to a jew or non-jew (Bava Kamma 37B). Because the decrees of the greeks were designed to remove the differences between Jews and non-Jews, they specifically made them write it on the horn of the animal. (B'nei Yissachar and Migaleh Amukos)

The Gemara (Bava Kamma 2B) says that when an animal gores it intends to inflict pleasure and derives no benefit from it. Similarly, the Greeks made decrees solely to inflict pain on the Jews. (Fun Unzer Alter Otzer)

See here and here for explanations connecting the horn to Yosef, who is called "B'chor Shor" (a firstborn ox).

I always thought it was just because oxen were commonly used as draft animals. So your average Jew would then see this message in front of him throughout his workday, and (the Greeks evidently hoped) would eventually internalize this message, G-d forbid.